Existing implementations of the amplitude-modulated, phase-shift measurement technique are capable of very high accuracy when the remote surface is a cooperative target such as a retro-reflective prism. Under these conditions, the reflected beam is of relatively high power when it is received at the instrument, enabling photoelectric detection of even very high frequency modulation to be achieved with a high signal to noise ratio. Under such ideal conditions, at higher modulation frequencies, the distance over which the phase difference between the transmitted and reflected beams cycles through 360 degrees becomes shorter and the greater is the resulting accuracy of distance measurement for a given accuracy of comparative phase measurement.
Simply, an intensity-modulated beam is transmitted from a measuring station, is reflected from a remote surface, and the reflected beam is received back at the measuring station. The phase difference between the transmitted beam and the reflected beam is used to determine the distance from the measuring station to the remote surface. The phase-shift φ or phase difference in degrees is equal to 360(2d/λ) and in radians is equal to 2π(2d/λ), where d is the distance to be measured and λ is the wavelength associated with the intensity modulation envelope (λ=c/F, where c is the velocity of light and F is the modulation frequency). As a simple example, for F=25×106 Hz, c=300×106 m/s and d=1.5 m the resulting phase-shift φ is 90 degrees or π/2. As is well understood, if d>λ/2, the phase difference exceeds 2π radians, leading to ambiguities in the inferred distance which may be resolved by changing the value of F and repeating the measurement. λ/2 is called the ambiguity interval.
However, when measurements are made on an optically rough surface, the received reflected beam is of low power, particularly if the power of the transmitted beam is limited to prevent any significant risk of damage to the eyes of an operator. Available optical detectors are incapable of measuring low power beams at high frequencies. Accordingly, under low power conditions, the characteristics of the available optical detectors limit the maximum practical modulation frequency, the maximum achievable signal to noise ratio and, consequently, adversely affect the accuracy of distance measurement that can be achieved. Conventional rough surface measurement systems based on modulation phase measurement have achieved ranging accuracies varying between 0.1 mm and several mm, using a measuring beam power typically in the region of 30-50 mW in conjunction with modulation frequencies from around 10 MHz to over 700 MHz. Note that maximum power output allowed for Class IIIA laser products is 5 mW.
Prior art implementations of intensity-modulated, phase-shift measurement techniques have used fast response optical detectors to sense the high frequency modulation of the received reflected beam. Electronic mixing techniques have then been applied to the high frequency electronic output from the detector so as to generate low frequency signals preserving the important phase information. However, if the optical detector is not capable of resolving the information from the beam, then the output will have a low signal to noise ratio.
There is a need for a system which circumvents the limitations of the available optical detectors so as to respond to the reality of low power reflected beams.